In U.S. Pat. No. 3,299,424 (Vinding) there is disclosed an interrogator-responder identification system in which the responder is identified when inductively coupled to the interrogator. The inductive coupling is achieved by means of resonant circuits tuned to the same frequency within the responder and interrogator, thereby enabling non-contact communication between the two.
In a preferred embodiment the responder is self-powered, deriving its dc supply voltage by rectifying a portion of the induced interrogator signal.
Data stored within the responder is read, or identified, by the interrogator by means of a detuning or loading circuit coupled to the responder through a switch means. The switch means is activated in response to the stored data so as to load the responder resonant circuit, thereby decreasing its interaction with the interrogator resonant circuit. Consequently, the varying loading effect of the responder on the interrogator resonator circuit may be interpreted in terms of the responder data. For example, a signal corresponding to the responder data may be transmitted to the interrogator by amplitude- or phase-modulating the resonant frequency signal of the interrogator.
Whilst Vinding discloses a system in which a responder, self-powered by means of a signal transmitted by an interrogator, transmits data to the interrogator, there is no provision for writing data from the interrogator to the responder.
U.S. Pat. No. 4,845,347 (McCrindle et al) discloses a transaction system permitting bi-directional communication between a portable token and a terminal. Both the terminal and the token include resonant circuits tuned to the same frequency and the token is self-powered by the energy transmitted by the terminal resonant circuit and received by the token resonant circuit by mutual coupling.
Data is transmitted from the token to the terminal by loading the token resonant circuit, thereby modifying its impedance and amplitude modulating the carrier frequency radiated by the terminal resonant circuit.
Data is transmitted from the terminal to the token by frequency modulating the carrier signal generated by the terminal resonant circuit. In particular, the two logic levels HIGH and LOW are transmitted from the terminal to the token by means of two different frequency signals. The token operates on one of two different resonant curves according to which of the two logic levels is received.
Thus, whilst McCrindle et al disclose a system for bi-directional communication between a fixed terminal and a portable token, resonant circuits must be provided in both the terminal and the token and data is transmitted from the terminal to the token by frequency modulation of the terminal resonant circuit carrier signal.
In U.S. Pat. No. 4,814,595 (Gilboa) there is disclosed a data transmission system for the non-contact transmission of data between a station and a portable data card, both of which contain resonant circuits tuned to the same frequency. The data card receives power from the station via inductive coupling of the two resonant circuits and transmits data to the station by means of a loading circuit on the card which loads the card resonant circuit and hence, by mutual coupling, the station resonant circuit in response to data stored within the card. The card also contains a reading circuit containing a pulse generation circuit which generates a pulse whenever the power received by the card resonant circuit is interrupted and then restored. Data may thus be transmitted from the station to the card by deactuating the station resonant circuit in response to the data to be transmitted, the resulting pulses generated within the card being interpreted as the transmitting data.
The pulse generation circuit employed by Gilboa is based on a JK flip-flop which changes state in response to an incoming clock pulse. According to the communications protocol employed by Gilboa, a clock pulse is produced by the card reading circuit whenever the station resonant circuit (and consequently the card resonant circuit also) is deactuated and subsequently re-energized. In such a communications protocol, each pulse generated by the card reading circuit may contain a plurality of logic "0" or "1" bits, the number of such bits corresponding to the time interval between deactuation of the station resonant circuit and its subsequent re-energization.
It is therefore necessary in the card reading circuit to sample the pulse train generated by the pulse generation circuit at very precise time intervals in order to reproduce the correct number of logic pulses between successive deactuations and subsequent re-energizations of the station resonant circuit. This requires the provision of a very precise timing circuit driven by a quartz crystal which is expensive and vulnerable to damage, particularly if the portable card is dropped or otherwise manhandled.
It will be appreciated that the prior art systems either require resonant circuits in both the station and the card for providing a resonant carrier signal which can be modulated with a data signal or, alternatively, require transformer coupling for two-directional data transfer.
However, an inherent problem associated with the use of a resonant circuit in the station is that, in order to operate at resonant frequency, the resonant circuit components, including the antenna and any cable connected thereto, have to be carefully calibrated. In practice, this requires that once the station resonant circuit has been assembled and tuned, the maximum displacement of the antenna relative to the station is fixed and cannot be altered without returning the resonant circuit. This is not always desirable for several reasons. First, the station may represent a secure data system, such as a bank and it may be desirable to locate the antenna a significant distance from the station itself. This is impractical in prior art systems because the capacitance of the connecting cable will be so large as to throw the resonant circuit out of resonance.
It should also be observed that in the prior art systems described above employing resonant circuits, data is generally transmitted from the card to the station by loading the card resonant circuit. This reduces the response of the card resonant circuit and, by mutual coupling, imposes a similar loading on the station resonant circuit and a consequent reduction in response thereof. When there is a significant amount of noise in such systems, the noise can swamp the reduced signal response. It is therefore preferable to detect transmitted data as a result of increased response rather than reduced response in order that the data signal can still be detected even in the presence of noise.
Conventional data transmission protocols for transmitting data from the card to the station in prior art systems include the Manchester code wherein a change in logic level from logic HIGH for a duration of T to logic LOW for a duration of T is interpreted as logic 0, whilst a change in logic level from logic LOW for a duration of T to logic HIGH for a duration of T is interpreted as logic 1. Thus, each pulse transmitted in the Manchester code occupies a time interval of 2T.
The protocol employed by Gilboa for transmitting data from the card to the station is illustrated in FIGS. 6a and 6b of the above-mentioned U.S. Pat. No. 4,814,595. A logic "0" is interpreted as a change in logic level from logic LOW for a time duration of 2T to logic HIGH for a time duration of T, whilst logic "1" is interpreted as a change in state from logic LOW for a time duration of T to logic HIGH for a time duration of 2T. Thus, in such a protocol each data bit occupies a time interval of 3T.
It would be desirable to employ a communications protocol wherein each logic bit occupies, on average, a time interval at least less than 2T as required by the Manchester code.